Weight paint in cycles nodes?
You need to translate vertex weight into some proxy that nodes can read.  Consider using UV as a proxy.  It can be read by nodes and it can easily be mapped to vertex group weight.

Create a new UV mapping for your mesh and set all vertices to 0,0 on that new map.  Add a UV warp modifier editing this new mapping, modulated by the vertex group you want to reference, and set it to use two empties, one located at 1,1 and one at 0,0.  Now, by accessing these new UV coordinates, you're accessing the vertex group modulating the modifier.